A study of manganese dioxide-hydrogen insertion compounds produced by different chemical insertion methods

The reduction of an electrolytic manganese dioxide (EMD) by two different
chemical insertion methods (propan-2-o1 and hydrazinc .hydrate) has been studied by
XRD (X-Ray Diffraction), FTIR (Fourier Transfonn infrared) and Electrode Potential
Me:lsurements.
The level of H insertion may be represented by MnOOH" w'here r goes fi-om :::: 0,1
in the starting material (due to non-stoichiometry) up to 1.0 in the most reduced material.
H inseniol1 into EMD led to approximate isotropic lattice expansion up to r values close
to 0.7 - O.S for the propan-2-01 reduction method. This observation was consistent with a
homogeneous solid state reduction with formation of a solid solution in which II:'" and eare
mobile. In the region r = 0,7-0,8 to LO, new XRD non-moving lines emerged. while
the original lines continued to move, indicating anisotropic lattice expansion. This is due
to the appe:lrance of microdomains of the end product within the solid solution, implying
that H- and e' were no longer mobile in the crystal structure bur located in position, which
has been supported by FTIR measurements looking carefully at wavenumber regions
where 0-H vibration occurred.
For the hydrazine hydrate reduced samples, the O-H bond formation takes place at
a much earlier stage of reduction. Examination of the XRD patterns indicated
heterogeneous solid state reduction had occurred after r = 0,4. Heterogeneous reduction
was presumed to have occurred by H location in the outside layers of the particles.
A study of the potential of the compounds obtained under both reduction methods
has been carried out. together with the study of the stability of the H inserted compounds
in KOH electrolyte. Potential measurements confilmed development of a heterogeneous
potential coinciding to the appearance of new peaks on X-ray diffi:action and the
f0l111ation of O-H bonds as shown in FTIR. This behaviour appears at Cl. much earlier level
of reduction for the hydrazine hydrate reduced samples than for the propan-2-01 reduced
samples. The results confilm previous findings that H inserted compounds are unstable at
a !evel related to the fOlmation of micro domains in KOH at concentrations similar to
those used in alkaline manganese batteries, which limits the capacity of those batteries.

Thesis submitted in partital fulfilment of the requirement for the degree of Master of Philosophy.